1. Field of the Invention
The present invention relates to a surface acoustic wave device, such as a surface acoustic wave resonator or a surface acoustic wave filter, and a method of manufacturing the same, and more particularly, the present invention relates to the structure of and a method of forming an electrode of a surface acoustic wave device.
2. Description of the Related Art
A conventional surface acoustic wave device is an electronic device utilizing a surface acoustic wave in which the propagation of mechanical vibration energy is concentrated near a solid surface. The surface acoustic wave device generally includes a piezoelectric substrate having piezoelectricity, and an electrode such as an interdigital electrode and/or a grating electrode, which is provided on the piezoelectric substrate, for applying an electrical signal.
Such a surface acoustic wave device generally includes an Al electrode material having low electrical resistivity and a low specific gravity, or an Al alloy composed of Al as a main component.
However, Al has low resistance to stress migration, and thus the supply of high electric power causes hillocks or voids in the electrode, resulting in the occurrence of a short-circuit or disconnection in the electrode. This leads to damage to the surface acoustic wave device in many instances.
To solve the above-described problem, a conventional method is disclosed in Japanese Unexamined Patent Publication Application No. 7-162255 (hereinafter xe2x80x9cfirst prior artxe2x80x9d), in which crystal orientation is improved by using an ion beam sputtering process as an electrode forming process to improve power endurance. Another method is disclosed in Japanese Unexamined Patent Publication Application No. 3-48511 (hereinafter xe2x80x9csecond prior artxe2x80x9d), in which a crystal is oriented in a desired direction by epitaxial growth of Al to improve power endurance.
However, the first prior art has insufficient power endurance for application to a radio frequency and high electric power.
The second prior art is applicable only to a quartz substrate, and thus causes difficulties in obtaining an epitaxial film with good crystallinity on a LiNbO3 or LiTaO3 substrate which has high piezoelectricity and is widely used for a filter, and other piezoelectric devices.
To overcome the above-described problems, preferred embodiments of the present invention provide a surface acoustic wave device including a piezoelectric substrate, and an electrode provided on the piezoelectric substrate, wherein the electrode has a twin crystal structure.
Also, other preferred embodiments of the present invention provide a surface acoustic wave device including a piezoelectric substrate, and an electrode provided on the piezoelectric substrate, wherein the electrode includes Al or an Al alloy including Al as a main component, and exhibits six-fold symmetry spots in a XRD pole figure.
In this surface acoustic wave device, the electrode preferably includes an Al electrode layer made of Al or an Al main component, the Al electrode layer having a crystal orientated in a desired direction such that the normal direction of a (111) crystal plane of Al substantially coincides with the Z crystal axis of the piezoelectric substrate.
In a further preferred embodiment of the present invention, a surface acoustic wave device includes a piezoelectric substrate made of a LiNbO3 or LiTaO3 single crystal, and an electrode provided on the piezoelectric substrate, wherein the electrode does not necessarily have a twin crystal structure. In the present invention, the electrode may include an Al electrode layer including Al as a main component, the Al electrode layer having a crystal orientated in a desired direction such that the normal direction of a (111) crystal plane of Al substantially coincides with the Z crystal axis of the piezoelectric substrate.
The electrode preferably further includes an under electrode layer provided between the Al electrode layer and the piezoelectric substrate, for improving the crystallinity of Al.
The under-electrode layer preferably includes at least one of Ti and Cr as a main component.
In this preferred embodiment of the present invention, the piezoelectric substrate is preferably a 64xc2x0 Y-X cut LiNbO3 substrate. However, other suitable substrates may be used.
In the surface acoustic wave device of various preferred embodiments of the present invention, with the electrode including the Al electrode layer, an electrically insulating protecting film is further provided to cover the surface and sides of the Al electrode layer.
In a further preferred embodiment of the present invention, a method of manufacturing a surface acoustic wave device is provided which includes a piezoelectric substrate made of a LiNbO3 or LiTaO3 single crystal, and an electrode provided on the piezoelectric substrate and includes an Al electrode layer made of Al or an Al main component, the Al electrode layer having a crystal orientated in a predetermined direction such that the normal direction of a (111) crystal plane of Al substantially coincides with the Z crystal axis of the piezoelectric substrate.
The manufacturing method preferably includes the step of preparing the piezoelectric substrate made of a LiNbO3 or LiTaO3 single crystal, the step of removing a damaged layer from the surface of the piezoelectric substrate, and the step of forming the electrode on the piezoelectric substrate, wherein the electrode forming step includes the step of forming an under-electrode layer made of at least one of Ti and Cr as a main component on the piezoelectric substrate by a vacuum deposition process at a temperature of about 100xc2x0 C. or less, and the step of forming an Al electrode layer made of Al or an Al main component on the under-electrode layer.
The manufacturing method preferably uses a 64xc2x0 Y-X cut LiNbO3 substrate. However, other suitable substrates may be used.